physics question #2050

James Hope, a 17 year old male from Gloucester asks on April 16, 2004,

Q:

I recently read a book on the big bang, and it said that protons and neutrons are made up of 3 quarks. It also said that up quarks have a + charge and down quarks have a - charge. If I am correct then a proton is made up of 2 ups and a down quark, this leaves a + charge. And a neutron is made up of 2 downs and a up quark, surely this would leave a - charge. Why are neutrons neutral if this is the case?

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the answer

Well, either the book left out an important aspect of quarks or you read too fast. The weirdest things about quarks is that their charges are not "unit" charge + or - (like a proton or an electron) but +2/3 and -1/3 respectively for up and ddown quarks. (I think I have that right.) Anyway, this explains how two up plus 1 down can have charge +1 while two down plus 1 up can have zero charge. (The proton & neutron respectively.) You might well ask, wouldn't particles with such weird charges be easy to detect? Probably they would, if you could ever "see" a single bare quark. But you can't, because the force between quarks gets BIGGER as they get further apart (almost like a spring -- but not quite) so by the time one quark starts to get off by itself, the vacuum between it and its compatriots is torn apart, releasing quark-antiquark pairs that even things up again. You might ask, then, are quarks really real? Good question. For decades people considered them to be just a cartoon version of a mathematical symmetry obeyed by the strong interaction; but it turned out to be such a GOOD cartoon (all aspects of the theory seem to be embodied in this picture) that the argument boils down to a quibble over what you mean by "real". Crazy stuff, eh?